The assignee of the instant patent application provides power distribution equipment meeting the military requirements of, for example, the U.S. Navy. The power distribution equipment includes electro-mechanical devices such as circuit breakers that must reliably operate in the face of large and unpredictable mechanical shocks and related environmental transients.
Circuit breakers are frequently equipped with undervoltage release (UVR) mechanisms. A UVR mechanically trips the circuit breaker to the open position, when an undervoltage event occurs. A UVR may be added to a circuit breaker for a number of reasons. For example, a UVR may provide protection for an electrical system which employs dual power inputs by opening a breaker associated with a power source that is off-line to prevent power back-feeding into that source from an on-line power source. Another possible application of a UVR is to provide an inexpensive type of coordination of the breakers. A UVR in a branch breaker will open that breaker when the main breaker trips, thus assuring that when power is restored, that branch breaker will remain open until some required action is taken.
The UVR must reliably trip the circuit breaker upon an onset of a specified low voltage condition, but must also be relied upon to avoid inadvertently tripping the circuit breaker as a result, for example, of a mechanical shock. Because a UVR must trip the breaker when there is no power in the system, the energy required to trip the breaker must be stored. This may be accomplished by storing energy in a helical coil spring, for example.
A conventional UVR, illustrated in FIG. 1, may include a linear actuator 1 employing a solenoid having a magnetically permeable armature 12, and an electromagnetic inductive coil 15. Coil 15 is wound about a magnetically permeable, cylindrical annular core 16 secured within frame 17. The interior of core 16 is sized to receive armature 12 and permit axial motion of armature 12. A helical coil spring 13 may be provided to bias the armature 12 in an extended position such that, in the absence of a countervailing force, the armature end 23 engages a circuit breaker trip button 27.
When a requisite amount of voltage (“holding voltage”) is provided to coil 15 by a power supply (not shown), the armature 12 is held in a retracted position by an electromagnetic field generated by coil 15, and armature end 23 is separated by some distance from trip button 27. When current is removed from coil 15, or the voltage from the power supply drops below the requisite holding voltage, the electromagnetic field becomes insufficient to overcome the bias provided by spring 13, whereupon the armature 12 moves to the extended position and armature 23 engages trip button 27.
The actuator described in general terms above, and many variants thereof, are well known in the art to be particularly sensitive to dynamic loads resulting from mechanical shock or vibration. This sensitivity results from the intersection of competing design imperatives. On the one hand, upon occurrence of an undervoltage event, armature 12 must be driven by spring 13 a sufficient distance, and with sufficient force, to successfully actuate trip button 27. On the other hand, the power required to hold armature 12 in the retracted position must be minimized in order to avoid, for example, unnecessary heating of coil 15. Thus, efficient designs provide that the normal holding voltage provided to coil 15 has minimal margin over that required to overcome the bias provided by spring 13. This presents a problem when mechanical load transients are encountered, because a relatively small shock pulse, for example, can cause armature 12 to start to move from the retracted position. Once that occurs, energy stored in spring 13 can easily cause actuator 1 to inadvertently and inappropriately actuate trip button 27.
The foregoing problem has been widely recognized in the art, and several methods directed toward mitigating it have been proposed.
One approach, disclosed in U.S. Pat. No. 6,317,308, proposes to detect an unscheduled movement of armature 12, by observing a change in value of current flowing in coil 15, the change in value resulting from electromotive force caused by the movement. Upon registering the change in value, a holding current to coil 15 is increased, with the objective of stopping or reversing the unscheduled movement, before an inappropriate actuation of trip button 27 occurs. But, since the holding current may only be increased after some measurable unscheduled movement occurs, there is a problem to mitigate the risk that the counteracting increase in holding current will be too late or of insufficient strength.
Other techniques, exemplified by U.S. Pat. Nos. 6,255,924, 6,486,758, and 7,486,164, for example, propose various additions to the basic linear actuator, intended to decrease its sensitivity to mechanical load transients. As proposed in U.S. Pat. No. 6,255,924, for example, resilient shock mounts are provided. U.S. Pat. No. 6,486,758 discloses an “inertial lock” that mechanically safes the linear actuator in the event of a shock event. U.S. Pat. No. 7,486,164 discloses various anti-shock devices intended to mechanically prevent movement of an armature in the face of a mechanical shock.
While the foregoing approaches mitigate to some extent the immediate problem of inappropriate actuation upon occurrence of an environmental transient, they increase the complexity, size and cost of the mechanism and/or may not be sufficient for severe military environments.
Accordingly, there is a long felt need for improved, acceptably shock tolerant UVR's, and similar linear actuators, that avoid the disadvantages of the known techniques.